A. Field of the Invention
This invention relates to the field of lightweight flowable and shearable materials including materials for cushions and padding, such as seat cushions or bed pads, where skin comfort and skin preservation are important. More particularly, this invention relates to a composite mixture of a lubricant and hollow spherical objects which may be encased within a flexible container or sack and used as a cushion or padding. The exterior surfaces of the spherical objects are lightly lubricated to accommodate flow and shear, but lubricant is not used in an amount sufficient to cause dispersion of the spherical objects in the lubricant. Additionally, the mixture has no memory which would cause it to move, flow or change shape absent a deforming force. The composite mixture has a very low specific gravity, making it an excellent flotation material.
B. The Background Art
In the prior art there has been substantial difficulty in designing padding or cushions for contact with a patient's skin which equalize pressure across the entire area of skin contacted, are flowable, have a low shearing force threshold, have no memory, are lightweight, do not substantially vary in performance with temperature, do not break down or separate into their constituent components over time, do not feel overly cold to the touch, and quickly warm to body temperature. The prior art has also been unable to design a flotation material which has a low specific gravity for buoyancy but was not stiff or semi-rigid and therefore somewhat uncomfortable for the wearer of a life vest containing such material.
For example, the typical prior art wheelchair seat consists of a fabric sling draped between two metal bars in which the patient sits. Frequently the sling has a plastic or rubber coating for durability. Sling seats such as this are unable to conform to the shape of the human body, resulting in greater force being applied to some portions of the patient's anatomy than to others. Over time, high spots or bony areas on the patient's body often develop abrasions, sores or callouses as a result. Sling seats typically also have a high shearing force threshold, being resistent to movement or turning in response to movement by the patient. As the patient turns or moves within the seat, friction with the patient's body would tend to move the portion of the seat in direct contact with the patient's body. All portions of the typical prior art seat, however, are typically held fast together, whether by monolithic construction, such as in foam seats, with stitching as in sling seats, or otherwise. The high resulting shearing force threshold causes small tears or abrasions on the patient's skin during turning or movement. Skin damage from shear-resistant seats can be cumulative and debilitating over time. Other traditional types of cushions and padding have provided little relief from the disadvantages of sling seats. For example, patients confined to bed typically experience the same skin damage as those using a sling seat even though mattresses usually include a combination of metal springs, foam padding and cotton or synthetic batting.
The best prior art solution to the problems of typical cushions and padding has been gels or viscous and slimy liquids (hereinafter "gels") contained within a flexible bag or bladder. The use of gels within a flexible bladder for contact with a patient's body achieves much more uniform force or pressure on irregular body surfaces than traditional prior art cushions and padding. The shearing force threshold of gel within a bladder may be lower than in other prior art cushions or pads because the bladder surface closest to the patient's body is free, within certain limits, to move with the patient's body as the patient moves. The bladder walls are not anchored to the gel so that as the patient moves, the bladder may move with respect to the gel. The gel also has some ability to shear with respect to itself. The result is a seat which is less damaging to human skin than a typical sling seat. Unfortunately, most prior art gels are very slow to respond to body movement, giving an undesirably high shearing force during quick body movement and failing to fully preserve the integrity of the patient's skin. Gel within a bladder is currently used as padding in some wheelchair and bicycle seats. Prior art gels are also very heavy, even when hollow microbeads are dispersed in the gel. Further, prior gels have memory, causing them to exert a constant force on bony protruberences of the body, causing skin damage and discomfort.
Gel has the disadvantage of being much heavier than the flexible foam used in typical cushions. Gels commonly used in cushioning have specific gravities of 0.8 to 1.2, whereas a typical cushion foam may have a specific gravity of only 0.05 to 0.10. Thus, in order to achieve a lightweight seat, gel must be used sparingly. When gel is used sparingly, its positive characteristics are minimized. Alternatively, gel may be used in combination with flexible foam. This is an undesirable compromise because much of the irregular pressure distribution sought to be avoided by using a gel is reintroduced. Use of gels in sufficient quantity to achieve the desired effect results in a seat weight unacceptable for many applications. For example, patients using wheelchairs commonly suffer from a weakened physical condition, and an increase in the weight of a wheelchair by only a few pounds can make it excessively burdensome for the patient to use. As another example, total bicycle weight is typically a primary consideration in the design of both road bikes and mountain bikes. The additional weight of a gel seat is often unacceptable to all but the most casual of riders.
Another disadvantage of prior art gels is that they have a high thermal mass and a high coefficient of heat transfer. As a result, gel seats typically feel cold to the touch, providing initial discomfort to the user and remain cold for long periods of time before warming. Gels used in prior art cushioning also exhibit substantial viscosity change with temperature, causing them to become more viscous and lose their functional characteristics in cold temperatures. Similarly, in very warm temperatures, prior art gels will exhibit excessive flow characteristics and again not function as desired. Finally, prior art gels used in cushioning will tend to break down and separate into their constituent components over time, causing a loss of functional characteristics. The user of a gel cushion must therefore periodically knead the cushion to re-mix the components of the gel and keep it functioning. Kneading a gel cushion can be difficult or impossible for many wheelchair users because of the hand strength required, making a gel cushion very undesirable.
Prior art U.S. Patents to Terrence M. Drew et al. issued Mar. 3, 1992, (No. 5,093,138), and Mar. 31, 1992, (No. 5,100,712), describe a flowable, pressure compensating composition including water, a material for increasing the viscosity of water, and spherical particles dispersed throughout the volume of the water. Both of these patents are hereby incorporated by reference in their entirety. The composition disclosed in these patents is a deformable gel which has the disadvantages of substantial weight, memory, and being slow to flow or shear in response to a deforming pressure, thus being prone to cause skin damage.
Prior art U.S. Patents to Chris A. Hanson issued Oct. 22, 1991, (No. 5,058,291) and Aug. 28, 1990, (No. 4,952,439), describe padding devices which are resistant to flow in response to an instantly applied pressure. These two patents are hereby incorporated by reference in their entirety. The padding disclosed in these patents has the disadvantage of being slow to flow in response to pressure, thus having a high shearing force and being prone to cause skin damage. The materials disclosed in these patents also have memory, causing them to tend to return to their original shape after removal of a deforming pressure. The memory of the material causes the material to exert additional pressure on protruberences of the body, such as on bony areas, increasing the risk of skin abrasions and sores. Typically, material having memory has a high shear force as well, increasing skin damage during use. Memory is described in U.S. Patents to Chris A. Hanson issued Sep. 15, 1992, (No. 5,147,685), Terrence M. Drew issued Apr. 20, 1993, (No. 5,204,154), Chris A. Hanson issued Aug. 28, 1990, (No. 4,952,439), Terrence M. Drew et al. issued Mar. 3, 1992, (No. 5,093,138), Chris A. Hanson issued Oct. 22, 1991, (No. 5,058,291), and Terrence M. Drew, et al. issued Mar. 31, 1992, (No. 5,100,712), each of which is incorporated by reference in its entirety.
Prior art U.S. Patents to Eric C. Jay issued Mar. 1, 1988, (No. 4,728,551), Jack C. Swan, Jr. issued Jan. 6, 1981, (No. 4,243,754), Jack C. Swan, Jr. issued Jan. 6, 1981, (No. 4,229,546), Jack C. Swan, Jr. issued Jan. 6, 1981, (No. 4,255,202), Jack C. Swan, Jr. issued Jan. 6, 1981, (No. 4,144,658), Jack C. Swan, Jr. issued Jan. 6, 1981, (No. 4,038,762), Reginald Dyson issued Jul. 13, 1976, (No. 3,398,530), Henry Wilfred Lynch issued Oct. 19, 1976, (No. 3,398,213) and Frederick L. Warner issued Jul. 31, 1973, (No. 3,748,669), each of which is hereby incorporated by reference in its entirety, disclose pressure compensating mixtures which are generally characterized by having a quantity of microbeads dispersed in a flowable liquid medium. Disadvantages of such mixtures include their weight and memory. The liquid described in those prior art patents is formulated for certain flow characteristics and the microbeads are merely added because of their low specific gravity to reduce the total weight of the mixture. The resulting mixture is still very heavy because the light microbeads are not used to replace a substantial amount of the heavy liquid, but are instead used only to provide a slight weight reduction of the mixture compared to the use of a liquid alone.
Background information related to the invention is described in U.S. Patents to Terrence M. Drew issued Apr. 20, 1993, (No. 5,204,154), John C. Dinsmoor issued Apr. 13, 1993, (No. 5,201,780), Chris A. Hanson issued Sep. 15, 1992, (No. 5,147,685), Adrian Q. RoJas issued Jan. 14, 1992, (No. 5,079,786), Huibert P. M. Pollmann issued Jan. 14, 1992, (No. 5,079,787), Eric C. Jay issued May 28, 1991, (No. 5,018,790), Terrence M. Drew et al. issued May 14, 1991, (No. 5,015,313), Eric C. Jay issued Jun. 27, 1989, (No. 4,842,330), Eric C. Jay issued Aug. 9, 1988, (No. 4,761,843), Eric C. Jay issued Feb. 23, 1988, (No. 4,726,624), Donald H. Shaktman issued Dec. 1, 1987, (No. 4,709,431), Eric C. Jay issued Apr. 28, 1987, (No. 4,660,238), and Eric C. Jay issued May 13, 1986, (No. 4,588,229), each of which is hereby incorporated by reference in its entirety.
Prior art flotation devices, such as life vests, typically included a flotation material such as foam with small air bubbles throughout its interior. While such flotation material typically had a low specific gravity, it was typically semi-rigid or stiff and was uncomfortable to wear. The stiffness of the prior art flotation material caused prior art flotation devices such as life vests to restrict body movement of the wearer and result in discomfort. When a flotation device is made using the invented composite mixture, flotation qualities equal or superior to the flotation qualities of prior art flotation material is provided, but the deformable, flowable and shearable nature of the composite mixture permits it to respond to movement of body parts quickly and with little resistance resulting in a flotation device which does not restrict movement or activity. The composite mixture of the present invention has a further advantage as a flotation material because it may be inserted into containers of irregular shape and provide the containers with flotation characteristics. No forming of the composite mixture would be required as with prior art foam flotation materials.